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Capnography during thoracic anesthesia

Capnographic waveforms seen during thoracic anesthesia.

Bhavani Shankar Kodali MD

Abnormal CO2 waveforms that may be seen during thoracic anesthesia include:

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1). Capnograms withincreased phase III slope due to a large spread V/Q ratios, as in lung disease. The initial part of the slope is represented by areas which are well ventilated with high V/Q ratios (i.e., decreased CO2 concentration), while the latter part is represented by areas which are poorly ventilated and with low V/Q ratios (i.e., increased CO2 concentration).

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2.)Biphasic capnogram: ‘Phase III’ of the capnogram represents mixed alveolar gas at the CO2 sampling site. Therefore, if the lungs have distinctly different V/Q ratios and exhalation time constants, then a ‘Biphasic’ waveform can be seen, as in lateral decubitus position.1 The upper lung (non-dependent) has a low airway resistance, high V/Q ratio (secondary to gravity dependent blood flow) and a low CO2 concentration compared with the lower, dependent lung. The earlier part of the biphasic CO2 waveform is due to the expired gases from the upper lung containing lower PCO2 and the later part of the biphasic waveform is predominantly due to the expired gases containing high PCO2 from the lower lung.

A similar capnogram can occur following a single lung transplant Some patients with COPD may also display a slight biphasic expiratory plateau if they have, throughout both lungs, two distinct populations of alveoli with very different time constants. In this situation, rapidly exchanging alveolar spaces are overinflated during inspiration (their compliance is high) so that their CO2 concentration is low, whereas slower exchanging alveoli empty only during the later part of exhalation, releasing a higher CO2 content.1,2

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3.) Reverse phase III capnogram: Occasionally seen in patients with emphysema. The slope of phase III can be reversed in patients with emphysema where there is marked destruction of alveolar capillary membranes and reduced gas exchange.

Reference:

1. Benumof JL. Anesthesia For Thoracic Surgery. 2nd edition. W B Saunders company, 1995;245-50.

2. Carlon GC, Ray c, Miodownik S, Kopec I, Groeger JS. Capnography in mechanically ventilated patients. Crit Care Med 1988;67:579-81.

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